Vertical tube reactors which ordinarily involve the use of a subterranean U-tube configuration for providing a hydrostatic column of fluid sufficient to provide a selected pressure are well known. This type of reactor has been primarily used for the direct wet oxidation of materials in a waste stream and particularly for the direct wet oxidation of sewage sludge. Bower in U.S. Pat. No. 3,449,247 discloses a process in which combustible materials are disposed of by wet oxidation. A mixture of air, water and combustible material is directed into a shaft and air is injected into the mixture at the bottom of the hydrostatic column.
Lawless in U.S. Pat. No. 3,606,999 discloses a similar process in which a water solution or suspension of combustible solids is contacted with an oxygen-containing gas. Excess heat is removed from the apparatus by either diluting the feed with the product stream or withdrawing vapor, such as stream, from the system.
Land, et al. in U.S. Pat. No. 3,464,885 (issued Sept. 2, 1969) is directed to the use of a subterranean reactor for the digestion of wood chips. The method involves flowing the material through countercurrent coaxial flow paths within a well bore while flowing heated fluid coaxially of the material to be reacted. The reactants, such as sodium hydroxide and sodium sulfate, are combined with the wood chip stream prior to entry into the U-tube which is disposed within a well bore.
Titmas in U.S. Pat. No. 3,853,759 (issued Dec. 10, 1974) discloses a process in which sewage is thermally treated by limiting combustion of the material by restricting the process to the oxygen which is present in the sewage, i.e. no additional oxygen is added. Therefore, it is necessary to provide a continuous supply of heat energy to affect the thermal reactions.
McGrew in U.S. Pat. No. 4,272,383 (issued June 9, 1981) discloses the use of a vertical tube reactor to contact two reactants in a reaction zone. The method is primarily directed to the wet oxidation of sewage sludge in which substantially all of the organic material is oxidized. Heat exchange between the inflowing and product streams is contemplated. The temperature in the reaction zone is controlled by adding heat or cooling as necessary to maintain the selected temperature. It is disclosed that when gas is used in the reaction, it is preferred to use a series of enlarged bubbles known as "Taylor bubbles". These bubbles are formed in the influent stream and passed downward into the reaction zone. It is disclosed that preferably air is introduced into the influent stream at different points with the amount of air equalizing one volume of air per volume of liquid at each injection point. While such a large amount of oxygen can be needed to oxidize minor organic components dissolved or suspended in a primarily aqueous liquid, this process is not feasible when the liquid stream is primarily a mixture of hydrocarbons. The presence of such large volumes of oxygen could result in an uncontrollable exothermic reaction.
The above-cited patents which disclose vertical tube reactor systems describe the use of such systems with primarily aqueous streams. None of these patents describe treatment of a primarily hydrocarbon stream. Specifically, there is no suggestion of the thermal treatment of a hydrocarbon stream in a vertical tube reactor system.
The reduction in viscosity of heavy hydrocarbon material by thermal treatment are well known. The thermal cracking known as "visbreaking" involves the treatment of hydrocarbon materials at elevated temperatures and pressures. Such processes are exemplified by Biceroglu, et al. in U.S. Pat. No. 4,462,895 (1984), Beuther, et al. in U.S. Pat. No. 3,132,088 (1964), Taff, et al. in U.S. Pat. No. 2,695,264 (1954), and Shu, et al. in U.S. Pat. No. 4,504,377 (1985). Such processes are commonly used in refineries where there are the necessary distillation units to provide selective fractions to the visbreaking unit and the necessary product treatment facilities to handle the gaseous and low boiling products from the visbreaking unit. Such capital intensive processes do not readily lend themselves to the treatment of heavy oils at the production site to improve their transportability.
Co-pending and commonly assigned application U.S. Ser. No. 771,205 filed Aug. 30, 1985 now abandoned, discloses a method for viscosity reduction of a hydrocarbon feed in the field. In this process a vertical tube reactor is used to create a hydrostatic pressure on the crude oil feed and the feed is heated by an external heat source to provide the viscosity reduction necessary to improve transportability of the feed from the production area. The temperature differential between the heat source and the feed is maintained small to minimize the formation of coke.
Commonly assigned U.S. Pat. No. 4,648,964 of Leto et al. (1987) discloses the use of a vertical tube reactor to separate hydrocarbons from tar sands froth. The formation of coke deposits on the walls of the reaction vessels or heating surfaces has been a continuing problem. It has been disclosed that at higher severities there is an increased tendency to form coke deposits in the heating zone or furnace. Black in U.S. Pat. No. 1,720,070 teaches that operating at lower temperatures for increased lengths of time provides "a much smaller amount of carbon is deposited than is deposited at higher temperatures." Akbar et al., "Visbreaking Uses Soaker Drum", Hydrocarbon Processing, May 1981, p. 81 discloses that, when there is a high temperature differential between the tube wall in a furnace cracker and the bulk temperature of the oil, the material in the boundary layer adjacent to the tube wall gets overcracked and excessive coke formation occurs. In furnace cracking this boundary layer is commonly about 30.degree. C. to 40.degree. C. higher than the bulk temperature.
The problem associated with excessive coke formation in the boundary layer stems from the fact that the coke adheres to vessel walls. This coating of material acts to insulate the reaction vessel which necessitates additional heating for sufficient viscosity reduction. The added heat compounds the problem by further increasing coke formation.
In refinery operations, coke formation in viscosity reduction processes can be tolerated because frequent shutdowns of the process for coke removal are possible since storage space for the feedstock is usually available. However, this limitation is unacceptable in a field operation where crude is continually produced and must be rapidly transported. Such periodic shutdowns are also unacceptable with a vertical tube reactor system. In the co-pending application Ser. No. 771,205, the temperature difference between the heat source and the feed is kept small to minimize formation of coke. However, this process still has the limitation that the temperature of the wall of the reaction vessel is necessarily higher than the temperature of the bulk of the hydrocarbon stream. Consequently, over a period of time coke formation can occur which requires either a decoking operation or shutdown of the unit.
Accordingly, there is a need for an improved method for reducing the viscosity of recovered heavy hydrocarbon material in which coking of reactor vessels can be substantially reduced.
The present invention provides a method for reducing the viscosity of a hydrocarbon feed in which a final incremental amount of heat necessary for increased thermaly degradation of heavy components is provided by the exothermic oxidation of components in the feed. This process avoids undesirable coking in the reactor vessel by maintaining the temperature in the boundary layer of the stream near the vessel walls below coking temperatures.